Mesophotic Coral Ecosystems (MCEs) are the most understudied coral reef ecosystems in the world. MCEs are found in the tropics and subtropics between 30 and 150-m depths, a range defined by the limit of scientific scuba diving (30 m) and corals’ ability to photosynthesize (150 m) (Hinderstein et al., 2010). The mapping of mesophotic benthic habitats has documented distinct coral and algal substrates (Rooney et al., 2010; Blythe-Skyrme et al., 2013), but little data exist for the fish assemblages that inhabit these substrates, particularly in Hawai‘i. MCEs have been described as transition zones between shallow and deep-reef fish assemblages (Brokovich et al., 2008; Weijerman et al., 2019), and represent a frontier for research as demonstrated by the continuing discovery of new and conspicuous reef fish species (Pyle, 1996; Pyle et al., 2008; Rocha et al., 2017; Arango et al., 2019). Despite increasing interest in MCEs during the last decade (Lindfield et al., 2016; Abesamis et al., 2020), the effective management of this habitat remains problematic due to limited knowledge about the community structure and habitat associations of mesophotic fish assemblages (Dennis and Bright, 1988; Hinderstein et al., 2010; Bejarano et al., 2014; Pinheiro et al., 2016).

In Hawai‘i, MCEs have recently been described within select geographic regions in association with specific bathymetric habitat features (e.g., banks, ledges) or benthic cover (e.g., algal meadows, high coral cover) (Rooney et al., 2010; Blythe-Skyrme et al., 2013; Pyle et al., 2016). Many studies have investigated various aspects of MCE benthic habitats in the main Hawaiian Islands including coral beds, algal meadows, coral physiology, fish life histories, and fish trophic ecology (Boland and Parrish, 2005; Kahng and Kelley, 2007; Rooney et al., 2010; Blythe-Skyrme et al., 2013; Costa et al., 2015; Pochon et al., 2015; Bradley et al., 2016; Pyle et al., 2016; Spalding et al., 2016; Asher et al., 2017; Weijerman et al., 2019), with the number and spatial extent of surveys increasing in recent years (2009–2018) (Spalding et al., 2019). Much less is known of MCEs in the Northwestern Hawaiian Islands (NWHI) as a result of the additional logistical constraints of research at these distant locations. Seven studies conducted in the NWHI have been published to date, focusing on benthic substrate cover, fish assemblages on banks, a comparison of shallow reef vs. mesophotic fish, foraging habitat of the Hawaiian Monk Seal and/or levels of fish endemism (Parrish et al., 2000; Parrish and Boland, 2004; Rooney et al., 2010; Kane et al., 2014; Fukunaga et al., 2016, 2017; Kosaki et al., 2016).

Studies in Hawai‘i and elsewhere have described zonation in fish assemblages across mesophotic depths. Weijerman et al. (2019) divided fish assemblages of the mesophotic zone in Hawai‘i into an upper mesophotic zone (30–59 m) and a lower mesophotic zone (60–129 m) with deeper depths described as “rariphotic” and “bathyal” zones.

Mesophotic fish assemblages have been associated with various benthic habitats (Dennis and Bright, 1988; Bejarano et al., 2010; Garcia-Sais, 2010; Asher et al., 2017; Stefanoudis et al., 2019; Williams et al., 2019) and with depth (Brokovich et al., 2008; Pearson and Stevens, 2015; Semmler et al., 2016; Asher et al., 2017; Sih et al., 2017; Coleman et al., 2018; Stefanoudis et al., 2019; Williams et al., 2019) and observed as early as 1981 in the Cayman Islands (Lukens, 1981). Thresher and Colin (1986) perceived multiple “fish communities” within the mesophotic assemblage of fishes on the outer wall at Enewetak Atoll in the Marshall Islands; however, their categorization was not rigorously quantitative. Dennis and Bright (1988) recognized multiple benthic-fish associations that were related to depth at the Flower Garden banks in the northwestern Gulf of Mexico.

This study investigates the MCE reef fish assemblages of the Au‘au Channel between Lāna‘i and Maui in the main Hawaiian Islands using archived MCE survey data collected between 2007 and 2011 by the National Oceanic and Atmospheric Administration (NOAA) National Marine Fisheries Service (NMFS) These surveys utilized three distinct platforms: Towed Optical Assessment Device (TOAD), submersibles Pisces IV and V (SUB), and mixed-gas scuba diving (SCUBA).

Fifty-two surveys (9 SCUBA diver, 29 SUB and 14 TOAD) were conducted between 53 and 115 meters utilizing the three survey platforms (Table 1). Eighty-two fish species and an additional ten fish genera were identified (92 total taxa). Ten different benthic substrates were observed and classified as: sand, hard bottom, rubble, Leptoseris sp. coral, Montipora sp. coral, Porites sp. coral, black coral, Halimeda sp. algae, Microdictyon sp. algae, and Ulva sp. algae. There was a significant but weak, positive relationship between fish assemblage and depth or substrate (Mantel test, r = 0.200, P < 0.001, 999 randomizations).

NMDS yielded a significant (P < 0.001) 3-dimensional solution that altogether explained 50.4% of the variance (21.7% Axis 1: Depth, Leptoseris and Hard Bottom, 14.8% Axis 2: Montipora and 13.9% Axis 3: Halimeda) with a stress of 11.567. Using Clarke’s rule of thumb where 5–10 is good ordination and 10–20 is a usable portrayal, a stress of 11.567 indicates a low risk of misinterpretation (McCune and Grace, 2002; Figure 2). Pair-wise orthogonality of the 3 axes were 99.3% (axis 1 vs. axis 2), 99.3% (axis 1 vs. axis 3), and 99.7% (axis 2 vs. axis 3), indicating that all axes were aligned at nearly 90 degrees to each other. Mean depth, 2 substrate types, and 8 fish species were significantly correlated with Axis 1 (Table 2). Both age classes of Cirrhilabrus jordani but no substrates or depth, were significantly correlated with Axis 2. Finally, Halimeda substrate and Adult Oxycheilinus bimaculatus were significantly correlated with Axis 3 (Table 2).

NMDS ordination was used to identify discrete groupings of samples with similar fish community structure, using Hierarchical Clustering with a threshold of 50% for defining groupings (Figure 3). This produced ten clusters and three ungrouped surveys (Table 3 and Figure 3). These clusters, which were grouped primarily by fish species and secondarily by substrate types had descriptors assigned based upon this hierarchy of definition (Table 3). These different groups were then plotted on the NMDS ordination graph (Figure 2) and illustrated the relation of the groups with the substrate-defined axes.

The grouping pattern appeared to correlate well with the plots of substrate (Figure 4) and fish species (Figure 5) indicating fish assemblages were affected by substrate. There are two clusters of 7 surveys each that are defined by a damsel fish species (Chromis verater or Chromis leucura) and each species association with Leptoseris coral beds (Table 3). Another cluster of six surveys defines shallow Montipora coral beds as habitats with juvenile Cirrhilabrus jordani. A cluster of six surveys shows the relationship of adult Oxychelinus bimaculatus with Halimeda algal beds. A cluster of five surveys defined a transition zone that had adult Cirrhilabrus jordani as the dominant fish species. Clusters defined transition zones, sand and rubble zones and even hard bottom zones (Table 3). The clusters are grouped consistently in regards to their member surveys when plotted (Figure 2) and suggests a strong positive association with substrate. These clusters defined five primary benthic habitats: Sand, Hard Bottom, Leptoseris coral beds, Montipora coral beds and Rubble. While Rubble was never a dominant substrate, it accounted for a large percentage of benthic habitat especially in the transition zone. Three singular surveys had unusual fish abundance that impacted their compatibility with other groups. The two surveys that were dominated by large schools were submersible surveys. In one case at a depth of 99 m, 85% of the observed fish were Aphraeus rutilans. In the other case at a depth 93 m, 95% of the observed fish were Luzonichthys earlei. A TOAD survey recorded a single Oxychelinus bimaculatus at a depth of 73 m (Table 3).

MRPP showed that fish assemblages differed (P < 0.001) among the five dominant benthic habitat categories (Table 4). Pairwise comparisons of fish assemblages indicated similarities among the benthic habitat types, with Rubble being central, sharing similarities with Sand, Montipora sp. coral bed, and Leptoseris sp. coral bed fish assemblages, whereas the fish assemblages of Hard Bottom only related to those of Leptoseris sp. coral beds (Figure 6). Fish assemblages also significantly differed (P < 0.001) between the six depth bins (Table 5). However, pairwise comparisons indicated relationships among the four shallowest depth bins (50–95 m), with the fish assemblages of the 2 deepest depth bins (96–105 m, 106–115 m) showing no similarities to the shallower depth bins or to each other (Figure 7).

Four significant Indicator Species, with values greater than 50%, were documented (Table 6). Two species were indicators of the deepest depth category (106–115 m): Pseudanthias hawaiiensis and juvenile Parapercis schauinslandii. Another two species were indicators of a hard-bottom substrate: Pseudanthias hawaiiensis and Odontanthias fuscipinnis. Centropyge potteri, was an indicator of Leptoseris sp. coral beds.

NMDS and HCA discerned fish assemblage associations with benthic substrates and depth in our Au‘au Channel data. NMDS Axis 1 showed the strongest association with increasing depth, hard bottom, and Leptoseris coral cover, whereas Halimeda algal fields were associated with Axis 3. Surveys conducted throughout the Hawaiian Islands by Rooney et al. (2010), Blythe-Skyrme et al. (2013), Costa et al. (2015), and Pyle et al. (2016) corroborate these findings, with Leptoseris coral sp. coverage increasing between the depths of 75 and 110 m and then diminishing at greater depths. Halimeda algal fields are prevalent throughout the Au‘au Channel at 90–100 m depths (Rooney et al., 2010). Leptoseris coral sp. and hard-bottom substrates increased with depth and supported a diversity of fishes including highly abundant planktivores such as Chromis verater and C. leucura. Increases in specific fish trophic groups associated with increasing depth and hard bottom have been described by Thresher and Colin (1986) who saw an increase in planktivores and Asher et al. (2017) who observed greater abundance and species in mobile invertivores.

Oxycheilinus bimaculatus is a wrasse that has a preference for algal beds and rubble fields (Randall, 2007). Halimeda fields comprise much bottom area in the Au‘au Channel (Rooney et al., 2010) and are bordered by transition zones of rubble or sand. Both Oxycheilinus bimaculatus and Halimeda are associated with axis three as indicators of an algal field habitat.

Montipora coral beds, another prevalent substrate in Au‘au Channel (Rooney et al., 2010), occurred only between 50 and 70-m depths; this habitat was populated by small-bodied and juvenile fish, primarily by juvenile and adult Cirrhilabrus jordani. Montipora coral beds were not strongly associated with a specific NMDS axis; however, they were most positively associated (tau value = 0.127) with Axis 2 and both life stages of Cirrhilabrus jordani. Axis 2 thus represents the Montipora coral bed habitat found at 50–70 m.

Large algal beds became obscured in our comparison between fish assemblages and substrate because the surveys that had algal beds contained larger areas of sand, rubble, or Montipora coral. The dominant cover was represented by five groups: Sand, Rubble, Hard Bottom, and Leptoseris and Montipora corals. Rubble was the central substrate with Sand, Montipora, and Leptoseris all associated with Rubble, but with no inter-relationship between each other. This result indicates that rubble is a transitional substrate that borders all of these habitats. Leptoseris coral also was associated with Hard Bottom, most likely because Leptoseris usually occurred in areas that had exposed Hard Bottom (Rooney et al., 2010; Blythe-Skyrme et al., 2013). This is reflected by the axes on our NMDS plot: Axis 3 represents the Halimeda sp. algae (Sand) and transition zones (Rubble), whereas Axis 1 represents the separate Leptoseris coral and Hard Bottom habitats.

Fish assemblages thus were inter-related in a manner that created two groups. The first set of inter-relationships indicates a clear depth range of associated fish assemblages: 50–95 m. This depth pattern seems to form an “upper” mesophotic fish assemblage in the Au‘au Channel. Deeper than 95 m the assemblages are unrelated to each other or the “upper” group which may mean that these depths are either a transition zone to deep ocean fish assemblages or that they form a “lower” mesophotic reef fish assemblage. Pinheiro et al. (2016), Semmler et al. (2016), Baldwin et al. (2018), Rocha et al. (2018), and Weijerman et al. (2019) describe similar breaks in “upper,” “lower,” and “rariphotic” fish assemblages at similar depths.

Four fish species showed relationships with either depth or substrate, with one appearing as an indicator for both. Adult Pseudanthias hawaiiensis and juvenile Parapercis schauinslandi were indicators of the deepest depth range of 106–115 m. Parapercis schauinslandi primarily occupies the sand and rubble that is found to greater extents at this depth. The P. schauinslandi that occur here appear to be mostly juveniles or small adults, which might indicate that the deep sand fields at the edge of Leptoseris beds are locations for juvenile recruitment, analogous to what Brokovich et al. (2007) found for a pomancanthid angelfish in the Gulf of Aqaba.

Centropyge potteri, Pseudanthias hawaiiensis, and Odontanthias fuscipinnis were indicator species for Hard Bottom. Pseudanthias hawaiiensis and O. fuscipinnis are usually found in areas of high relief, such as steep slopes or walls (Randall, 2007), corresponding to our Hard Bottom category. Pseudanthias hawaiiensis is a known deep-water species (Randall, 2007) and Hard Bottom increased as depth increased, further linking this species to the substrate and depth strata. NMDS results associating Centropyge potteri with rugose Leptoseris coral beds are consistent with the species preference for coral reef habitat (Randall, 2007).

ISA and MRPP results are further consistent with the original NMDS analysis, where Pseudanthias hawaiiensis and Centropyge potteri positively associated with Axis 1, the axis of Depth (Pseudanthias hawaiiensis), Hard Bottom (Pseudanthias hawaiiensis), and Leptoseris sp. coral (Centropyge potteri). This result indicates that mesophotic fish assemblages are associated primarily with specific benthic habitats similar to what was observed in the Gulf of Mexico by Dennis and Bright (1988), and in Puerto Rico by Bejarano et al. (2010) and Garcia-Sais (2010).

In the Au‘au Channel of Hawai‘I we observed what appeared to be a distinct “upper” mesophotic fish assemblage and either a “lower” or transitional assemblage as observed by Pinheiro et al. (2016), Semmler et al. (2016), Baldwin et al. (2018), and Weijerman et al. (2019).

Our analysis (NMDS, HCA, MRPP, and ISA) indicate that the mesophotic fish assemblage of the Au‘au Channel can be divided into an “upper” (50–95 m) and “lower” (96–115 m) group with specific species associated with specific benthic habitats. This is similar to what Stefanoudis et al. (2019) observed with mesophotic fish assemblage variance associated primarily with depth followed by benthic habitat. Further studies of fish (and other) assemblage components of mesophotic reef ecosystems are needed to further determine zonation of assemblages with depth and/or benthic habitat.

The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.

No animals were sampled by this project and thus there was no requirement for ethical review and approval for an animal study.

RB, FP, and JR conducted the surveys. KH, ED, and RB conducted the data analysis. RB wrote the manuscript. All authors contributed to the article and approved the submitted version.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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